Flashing, i.e., intermittently or periodically illuminated, lights have long been used to provide visual warnings, and a considerable body of research has been compiled in the fields of physiology, psychology and engineering concerning human perception of flashing light (i.e. the ability of people to perceive and respond to flashing light). This field of study involves the study of psycho-visual or psycho-optical sensory phenomena.
It is known in the art that certain factors may be applied to the provision of a flashing warning light for improving the visibility of a flashing light, that is, for making a flashing light visible at a greater distance, and for enhancing the probability that people will not only see the flashing light, but will also react consciously to it.
For example, some studies have revealed that human visual perception of flashing light appears greatest when the light is flashed at a flash rate or frequency in the range of 3 to 10 flashes per second, with a flash duration of at least 0.05 seconds. For the flashing of a light to be perceived as discrete flashes, the flash rate or frequency must be below the so-called “flicker-fusion” frequency, that is the frequency above which a flashing light appears as a steady light (“persistence of vision”), this critical frequency being considered to be approximately 24-30 flashes per second. Flash rate or frequency is often described in terms of “flashes-per-second” (fps).
Different flashing lights are known for providing visual alert or warning lights, and have employed incandescent lamps, rare gas discharge lamps and, more recently, light emitting diodes as illumination means, with some associated control circuitry. However, each of these prior art illumination means has had its disadvantages. In particular, prior art flashing light devices have not provided effective light output with low power consumption (i.e. high efficiency) at desirable high flash rates, and could not do so without severely sacrificing device power consumption and reliability of the light source. Thus a problem in the prior art has been the inability to provide a reliable warning light having high brilliance with low power operation, and that is suitable for use in portable lightweight battery powered equipment.
For example, incandescent light sources have commonly been used in flashing warning lights. However, they often are not able to come to full brightness and to then cool off to extinction (i.e. turn on and off) within the higher optimum flash rate frequencies for attracting attention. Also, the flashing character of typical tungsten-filament lamps is degraded significantly above flash rates of about 9 fps. Furthermore, because of the inherent thermal inertia of incandescent light sources (once turned sufficiently on to emit light, there is a significant delay in extinction to the off state), such light sources cannot provide flashes of relatively short duration, nor can such light sources provide adequate on-off contrast when operated at higher flash rates. In addition, an incandescent flashing light with adequate intensity for outdoor use usually requires larger size batteries to compensate for the excessive power loss in the form of heat, thus rendering it impractical for applications requiring reasonably small size and light weight necessary for portability. As a consequence, incandescent light sources are not suitable for use as warning lights at those flash rates and flash duration periods to which human visual perception is most sensitive but are constrained to use at lower frequencies and longer flash periods.
An alternative in the prior art has been rare gas discharge lamps, e.g., Xenon or Argon flash tube lamps and strobes. While such devices are capable of operation at higher flash rates they are also limited to extremely short flash durations which cannot be lengthened. Thus, such rare gas discharge light sources are incapable of longer flash duty cycle operation. Furthermore, rare gas discharge lamps are relatively expensive and must necessarily be energized with high voltages and currents, and thus flashing warning lights of this type require complex charging and discharging circuits and consume considerable power. In addition, a large amount of energy is required to produce the flashing action of a rare as lamp; thus tending to deplete ordinary batteries quickly if flashed at an optimal frequency of 3 to 12 Hz continuously such as that required by a warning light. As a consequence of these drawbacks, rare gas discharge light sources for extended flashing time are only feasible where a large power source is available, such as the utility power, or a power generator, but not in a portable application.
Light-emitting diodes (LED's) are well known semiconductor devices in which an electrical current is passed through a diode junction and produces light emission in an active layer of semiconductor material at the junction. At least one facet of the device is coated with an anti-reflective material, through which light is emitted. Ordinary LED's are relatively durable mechanically and electrically and, heretofore have most readily lent themselves to low voltage-low current operation and electronic control for both flash rate frequency and duration. However such ordinary LED's as have previously been used as light sources in flashing warning lights were of insufficiently low light intensity output. Hence the use of such low luminosity light emitting sources in visual warning devices has been of limited effectiveness, being restricted to subdued light environments such as for indoor activities, or where the ambient or background light level is quite low so that sufficient contrast can be obtained with the relatively dim illumination intensity of ordinary LED's to render them visible against a background. Thus, ordinary LED flashers have found wide application in toys, jewelry and traffic directional systems where visibility requirements are not critical.
One example may be found in U.S. Pat. No. 5,313,187, issued to Choi et al., where a one or more superluminescent light emitting diodes (SLEDS) are driven with an oscillatory square wave pulse drive signal varying between zero and about three VDC at a frequency between one Hz to twelve Hz, and having a pulse duty cycle between 5% to 10%. This arrangement periodically forward biases the SLED's into illumination, thus producing a brilliant rapidly flashing light. A low frequency oscillator stage is provided to generate an oscillatory square wave voltage signal Vo which drives a power driver stage to produce the correspondingly oscillating drive voltage signal Vd which is supplied to the SLED's. Significantly, the frequency and duty cycle of the drive pulse signal Vd are chosen to produce enhanced SLED illumination brightness and to operate the SLED within its most efficient operating characteristics. An exemplary circuit is provided that utilizes an astable monovibrator employing two transistors operated in the saturation mode with positive feedback as the low frequency oscillator, and a third transistor that is driven as a saturated switch by the oscillator output Vo. This acts as a power driver stage to switch battery current supplied to the SLED's as the drive voltage Vd for flashing the SLED's on and off at the frequency and pulse duty cycle of Vo. The pulse on time and off time and thus the flash frequency and duty cycle are determined by RC time constants of feedback circuits in the oscillator stage.
Prior art devices, while adequate for their intended purpose, suffer rom the common deficiencies associated with flashing light devices. In order to be oth effective and practical, a portable warning light should satisfy several requirements. It must provide adequate visibility and attention-getting luminous intensity as well as, adequate on-off contrast ratio of the light source, flash rate/frequency, and flash duration/period. It should be highly controllable, providing relative ease of control of the light source for effective flash rate frequency and flash duration. It should be driven by a systems that offers extended operating battery life, which requires balancing the interdependent factors of power available, light output intensity, and permissible weight of the device. It should also be light weight, small size, and capable of being retrofit into existing signaling and warning equipment currently in the field.
Thus, it remains desirable to provide a battery-powered flashing safety warning light which is simple and economical to manufacture and which is able to deliver effective illumination levels with high on-off contrast for high visibility and attention-getting performance while still providing long battery life.